The latest Bitcoin rally has been fueled by its biggest holders, according to a report from Kraken Intelligence, details an article in Bloomberg. “Whales”—holders of 100 Bitcoin or more—raised their average holdings 0.25% since early October, hitting a record $724.4 billion. Likewise, the amount of holders also increased 1.6% to over 16,000.

In the past year, Bitcoin has more than quadrupled, and the whales have grown more confident and remain optimistic about its future, especially with the recent debut of Bitcoin ETFs in the U.S., and decreased concern over issues like China’s regulatory actions on the digital-asset space. But, the article contends, many industry experts remain cautious about crypto, which remains very volatile.

Additionally, the article points to a recent report from the National Bureau of Economic Research that says the top 10,000 Bitcoin investors hold more than 1/3 of all the cryptocurrency in circulation, leading to concerns about market concentration.